Conventional engineering stress tests are often used for determining stress, strain, yield strength, tensile strength, fracture point, ductility, and other mechanical properties of parts and materials. These standard tests (per ASTM E8 and A370) do not take into account the “in test” reduction in cross sectional area as the specimen is stretched (a phenomenon called “necking”) and only uses the original gage area for generation of the engineering stress-strain curve. “In test” measurement of the actual cross sectional area is of key importance in the calculation of the “true stress−true strain” curve where measurement of this reduced area (i.e. necked region) is difficult to consistently and accurately measure in real time all the way to the point of specimen failure.
Conventional stress and strain testing systems also only measure the area of the specimens using stationary tags and/or digital markers usually in a single direction or single measuring position. Hence, these systems fail to correctly adjust to the correct area as these tags cannot move and are not usually placed at the location of minimum area throughout the test. This results in significant errors in the calculation of the true stress−true strain curve for the specimen being tested.